Online Test — Magnetic Effects of Current and Magnetism
20 Questions • 15 min • Chapter MCQ
15:00
Question 1 of 20
The Biot-Savart law gives the field $dB$ as proportional to:
$r^2$
$\frac{\sin\theta}{r^2}$
$\frac{1}{\sin\theta}$
$r$
Explanation: $dB=\frac{\mu_0}{4\pi}\frac{I\,dl\sin\theta}{r^2}$.
Question 2 of 20
The field at a perpendicular distance $a$ from a long straight wire is:
$\frac{\mu_0 I}{2R}$
$\frac{\mu_0 I}{2\pi a}$
$\mu_0 n I$
$\frac{\mu_0 N I}{2\pi r}$
Explanation: Ampere's law gives $B=\frac{\mu_0 I}{2\pi a}$.
Question 3 of 20
The field at the centre of a circular coil of $N$ turns and radius $R$ is:
$\frac{\mu_0 N I}{2R}$
$\frac{\mu_0 I}{2\pi R}$
$\mu_0 N I$
$\frac{\mu_0 I}{R}$
Explanation: $B=\frac{\mu_0 N I}{2R}$ at the centre.
Question 4 of 20
Ampere's circuital law is $\oint\vec{B}\cdot d\vec{l}=$
$\mu_0 I_{enc}$
$\frac{q}{\varepsilon_0}$
$\varepsilon_0 E$
$\mu_0 n$
Explanation: It equals $\mu_0$ times the enclosed current.
Question 5 of 20
The field inside a long solenoid is:
$\frac{\mu_0 I}{2\pi a}$
$\mu_0 n I$
$\frac{\mu_0 I}{2R}$
zero
Explanation: Uniform field $B=\mu_0 n I$ inside a long solenoid.
Question 6 of 20
Outside a long solenoid the magnetic field is:
very large
nearly zero
equal to inside
infinite
Explanation: The field outside a long ideal solenoid is essentially zero.
Question 7 of 20
The magnetic force on a charge moving with velocity $\vec{v}$ is:
$q\vec{E}$
$q(\vec{v}\times\vec{B})$
$qvB^2$
$\frac{qv}{B}$
Explanation: $\vec{F}=q(\vec{v}\times\vec{B})$.
Question 8 of 20
The magnetic force on a charge moving parallel to $\vec{B}$ is:
maximum
zero
half maximum
negative
Explanation: $F=qvB\sin\theta=0$ when $\theta=0^\circ$.
Question 9 of 20
The radius of a charged particle's circular path in a field is:
$\frac{qB}{mv}$
$\frac{mv}{qB}$
$qvB$
$\frac{2\pi m}{qB}$
Explanation: $qvB=\frac{mv^2}{r}\Rightarrow r=\frac{mv}{qB}$.
Question 10 of 20
The cyclotron period $T=\frac{2\pi m}{qB}$ is independent of:
mass
charge
speed of the particle
field
Explanation: $T$ does not depend on the speed or radius.
Question 11 of 20
The force on a current-carrying wire is given by:
$q\vec{v}\times\vec{B}$
$I\vec{L}\times\vec{B}$
$NIAB$
$\mu_0 n I$
Explanation: $\vec{F}=I\vec{L}\times\vec{B}$.
Question 12 of 20
Two parallel wires carrying currents in the same direction:
repel
attract
do not interact
twist
Explanation: Parallel currents attract.
Question 13 of 20
The torque on a current loop of magnetic moment $m$ in a field $B$ is:
$mB$
$m/B$
$mB\sin\theta$
$m+B$
Explanation: $\tau=NIAB\sin\theta=mB\sin\theta$.
Question 14 of 20
A moving-coil galvanometer uses a radial field so that:
the coil stops
the deflection is proportional to current
no torque acts
current is zero
Explanation: A radial field keeps $\sin\theta=1$, giving $\phi\propto I$.
Question 15 of 20
A galvanometer is converted to an ammeter by connecting a:
high resistance in series
low resistance (shunt) in parallel
capacitor
inductor
Explanation: A low-resistance shunt in parallel makes an ammeter.
Question 16 of 20
The axial field of a short bar magnet varies with distance as:
$1/r$
$1/r^2$
$1/r^3$
$r^3$
Explanation: $B_{axial}=\frac{\mu_0}{4\pi}\frac{2m}{r^3}\propto1/r^3$.
Question 17 of 20
The angle of dip at the magnetic equator is:
90 degrees
45 degrees
0 degrees
30 degrees
Explanation: At the magnetic equator the field is horizontal, so $I=0^\circ$.
Question 18 of 20
A diamagnetic material has susceptibility that is:
large positive
small positive
small negative
infinite
Explanation: Diamagnetics have small negative $\chi$ and $\mu_r<1$.
Question 19 of 20
Paramagnetic susceptibility obeys Curie's law:
$\chi\propto T$
$\chi\propto 1/T$
$\chi\propto T^2$
$\chi$ constant
Explanation: For paramagnets $\chi\propto1/T$.
Question 20 of 20
The area of a hysteresis loop represents the:
coercivity
energy lost per cycle
retentivity
permeability
Explanation: Loop area equals the energy dissipated per magnetisation cycle.